The manufacture of paper or paperboard involves the processing of an aqueous pulp fiber suspension, often referred to as the "furnish" to produce a uniform dry paper sheet. Numerous additives are used to treat the furnish which affect the final sheet properties of the finished paper. For example, pigments, sizing agents, fillers, and the like are commonly added to the furnish to improve brightness, opacity, color and/or ink receptivity. Other common additives include starches, polymers, china clay, titanium dioxide, optical brighteners, and the like.
Retention is a term used in papermaking to denote the extent to which the pulp fibers, contaminants and papermaking additives which are added to the furnish are retained in the finished paper. The retention of pulp fibers, sizing agents, fillers, anionic trash, other anionic contaminants from recycle paper and/or other additives in the paper sheet during its formation in a paper making machine is an important requirement to paper makers. A retention aid generally acts by increasing the flocculating tendency of the pulp fibers, contaminants and additives to inhibit their loss during drainage through the paper machine wires or screens. A high degree of retention is advantageous; however, too strong flocculation may be disadvantageous as it may result in poor paper appearance.
Numerous factors affect the efficiency of retention aids including 1) variables in the furnish such as pH, consistency, temperature, type of pulp fiber (e.g., fiber length, degree of refining, etc.), accumulation of contaminants and white water recirculation (e.g. degree of system closure), 2) conditions of the wire or screens such as wire mesh size, machine speed, etc. and 3) factors relating to the additives such as the dosage amount of additives, order of additives, form, shape and density of particles and ionic balance.
Another papermaking requirement that often conflicts with retention is the need for rapid drainage of the aqueous pulp suspension in the sheet forming areas of a paper machine. Aqueous pulp suspensions (i.e. the furnish) contain more than 99% water. To convert an aqueous pulp suspension to a finished paper sheet requires a rapid reduction in water content to a level of about 6%. Water removal is generally effected in three stages, a gravity or vacuum stage, a pressing stage, and a drying stage. The first stage in this process is commonly referred to as the formation stage which is where water is removed from aqueous pulp suspension, containing fibers and additives, using a wire screen. Drainage and retention aids are used to aid in water removal during this stage of the process. Dryness is typically in the order of about 20% after the formation stage. Water drainage rate during formation is dependent upon numerous factors including the arrangement of the drainage elements in the paper making machine, (e.g., arrangement of free drainage areas vis-a-vis vacuum assistance area), characteristics of the wires, screens or fabric, furnish characteristics (e.g. freeness, additives, etc.), furnish thickness, temperature, furnish consistency and wire speed. Suitable retention/drainage aids must not only inhibit the undue loss of fibers and additives, but they must also promote rapid drainage of water from the pulp suspension. Numerous retention/drainage aids are known and are available to paper makers.
For example, EP 235,893 describes the use of a combination of organic, substantially linear synthetic polymers and bentonite to improve drainage/retention. Suitable organic polymers are those which provide a bridging mechanism for flocculation. Suitable polymers in this reference require charge densities from 0.35 to 2.5 mEq/g, and molecular weights above 500,000, preferably above 1 million and often above 5 million, most preferably in the range 10 to 30 million or more. However, these high molecular weight polymers are provided to paper makers in the form of a solid material or in emulsion form. The solid form of high molecular weight polymers are generally slow to dissolve in aqueous systems. In addition, these high molecular weight polymers are very shear-sensitive, which presents significant handling and quality control problems. For example, the polymers must be pre-dissolved in an aqueous solution, which requires extensive mixing to assure complete dissolution. However, since these polymers are sensitive to shearing, these mixing procedures often destroy the desirable high molecular weight characteristics as well as overall uniformity of the final polymeric dispersion. In consideration of these problems, investigations into the use of lower molecular weight polymers for use as flocculating agents have been made, however, current low molecular weight polymers e.g., below 500,000 molecular weight, have been found to be relatively ineffective flocculating agents.
U.S. Pat. No. 4,749,444 discloses a process for production of paper and cardboard by adding to the paper stock a three component mixture of an activated bentonite, a cationic polyelectrolyte, having a charge density not less than 4 mEq/g, and a high molecular weight acrylamide or methacrylamide polymer having an average molecular weight from 1 to 20 million. This reference discloses that if only bentonite and cationic polyelectrolyte are used, the drainage of the paper stock is poor and if only bentonite and high molecular weight polymer are used, the paper stock flocculates to such an extent that satisfactory sheet formation is not ensured. Thus, in accordance with this reference effective flocculation for drainage/retention purposes, requires the presence of bentonite, a cationic polyelectrolyte and a high molecular weight (meth) acrylamide polymer.
U.S. Pat. No. 5,098,521 describes a process for paper and paperboard production which uses a paper stock which contains anionically charged foreign substances and comprises adding to the paper stock N-vinylamide/vinylamine copolymers having less than 10% vinylamine units and having K values of not less than 130 (as determined according to H. Fikentscher).
U.S. Pat. No. 4,880,497 to Pfohl et al discloses a process for making paper having high dry and wet strength which comprises applying to a paper sheet a hydrolyzed copolymer formed by copolymerizing (a) N-vinylformamide with (b) an ethylenically unsaturated monomer selected from the group consisting of vinyl acetate, vinylpropionate, C.sub.1 -C.sub.4 alkyl vinyl ethers, N-vinylpyrrolidone and esters, nitriles and amides of acrylic acid and methacrylic acid.
U.S. Pat. No. 4,808,683 to Itaqaki et al discloses vinylamine copolymers of N-vinylamine and comonomers having the structure ##STR4##
wherein R.sup.1 is hydrogen or methyl, R.sup.2 is hydrogen or C.sub.1 to C.sub.4 alkyl, and R.sup.3 is C.sub.1 to C.sub.4 alkyl or a C.sub.4 to C8 oxoalkyl group.
European patent application, publication No. 0 528 409 A, discloses cationic polymer flocculating agents having an amidine structure prepared from copolymerizing vinylformamide and acrylonitrile and heating in the presence of a strong acid to hydrolyze the polymers and form the amidine structure.
U.S. Pat. No. 5,324,792 to Ford discloses amine functional polymers which are terpolymers containing units of amidine or amidinium formate, N-vinylformamide and either vinylamine or vinylammonium formate.